Re: [dpdk-dev] [PATCH v2] mbuf:rearrange mbuf to be more mbuf chain friendly

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Keith Wiles
> Sent: Saturday, June 25, 2016 4:56 PM
> To: dev@dpdk.org
> Subject: [dpdk-dev] [PATCH v2] mbuf:rearrange mbuf to be more mbuf chain friendly
> 
> Move the next pointer to the first cacheline of the rte_mbuf structure
> and move the offload values to the second cacheline to give better
> performance to applications using chained mbufs.
> 
> Enabled by a configuration option CONFIG_RTE_MBUF_CHAIN_FRIENDLY default
> is set to No.

First, it would make ixgbe and i40e vector RX functions to work incorrectly.
Second, I don't think we can afford to allow people swap mbuf fields in the way they like.
Otherwise we'll end-up with totally unmaintainable code pretty soon.
So NACK.  

Konstantin

> 
> Signed-off-by: Keith Wiles <keith.wiles@intel.com>
> ---
>  config/common_base                                 |  2 +
>  .../linuxapp/eal/include/exec-env/rte_kni_common.h |  8 +++
>  lib/librte_mbuf/rte_mbuf.h                         | 67 +++++++++++++++-------
>  3 files changed, 56 insertions(+), 21 deletions(-)
> 
> diff --git a/config/common_base b/config/common_base
> index 379a791..f7c624e 100644
> --- a/config/common_base
> +++ b/config/common_base
> @@ -405,6 +405,8 @@ CONFIG_RTE_LIBRTE_MBUF_DEBUG=n
>  CONFIG_RTE_MBUF_DEFAULT_MEMPOOL_OPS="ring_mp_mc"
>  CONFIG_RTE_MBUF_REFCNT_ATOMIC=y
>  CONFIG_RTE_PKTMBUF_HEADROOM=128
> +# Set to y if needing to be mbuf chain friendly.
> +CONFIG_RTE_MBUF_CHAIN_FRIENDLY=n
> 
>  #
>  # Compile librte_timer
> diff --git a/lib/librte_eal/linuxapp/eal/include/exec-env/rte_kni_common.h b/lib/librte_eal/linuxapp/eal/include/exec-
> env/rte_kni_common.h
> index 2acdfd9..44d65cd 100644
> --- a/lib/librte_eal/linuxapp/eal/include/exec-env/rte_kni_common.h
> +++ b/lib/librte_eal/linuxapp/eal/include/exec-env/rte_kni_common.h
> @@ -120,11 +120,19 @@ struct rte_kni_mbuf {
>  	char pad2[4];
>  	uint32_t pkt_len;       /**< Total pkt len: sum of all segment data_len. */
>  	uint16_t data_len;      /**< Amount of data in segment buffer. */
> +#ifdef RTE_MBUF_CHAIN_FRIENDLY
> +	char pad3[8];
> +	void *next;
> 
>  	/* fields on second cache line */
> +	char pad4[16] __attribute__((__aligned__(RTE_CACHE_LINE_MIN_SIZE)));
> +	void *pool;
> +#else
> +	/* fields on second cache line */
>  	char pad3[8] __attribute__((__aligned__(RTE_CACHE_LINE_MIN_SIZE)));
>  	void *pool;
>  	void *next;
> +#endif
>  };
> 
>  /*
> diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
> index 15e3a10..6e6ba0e 100644
> --- a/lib/librte_mbuf/rte_mbuf.h
> +++ b/lib/librte_mbuf/rte_mbuf.h
> @@ -765,6 +765,28 @@ typedef uint8_t  MARKER8[0];  /**< generic marker with 1B alignment */
>  typedef uint64_t MARKER64[0]; /**< marker that allows us to overwrite 8 bytes
>                                 * with a single assignment */
> 
> +typedef union {
> +	uint32_t rss;     /**< RSS hash result if RSS enabled */
> +	struct {
> +		union {
> +			struct {
> +				uint16_t hash;
> +				uint16_t id;
> +			};
> +			uint32_t lo;
> +			/**< Second 4 flexible bytes */
> +		};
> +		uint32_t hi;
> +		/**< First 4 flexible bytes or FD ID, dependent on
> +			PKT_RX_FDIR_* flag in ol_flags. */
> +	} fdir;           /**< Filter identifier if FDIR enabled */
> +	struct {
> +		uint32_t lo;
> +		uint32_t hi;
> +	} sched;          /**< Hierarchical scheduler */
> +	uint32_t usr;	  /**< User defined tags. See rte_distributor_process() */
> +} rss_hash_t;
> +
>  /**
>   * The generic rte_mbuf, containing a packet mbuf.
>   */
> @@ -824,28 +846,31 @@ struct rte_mbuf {
>  	uint16_t data_len;        /**< Amount of data in segment buffer. */
>  	/** VLAN TCI (CPU order), valid if PKT_RX_VLAN_STRIPPED is set. */
>  	uint16_t vlan_tci;
> +#ifdef RTE_MBUF_CHAIN_FRIENDLY
> +	/*
> +	 * Move offload into the second cache line and next in the first.
> +	 * Better performance for applications using chained mbufs to have
> +	 * the next pointer in the first cache line.
> +	 * If you change this structure, you must change the user-mode
> +	 * version in rte_kni_common.h to match the new layout.
> +	 */
> +	uint32_t seqn; /**< Sequence number. See also rte_reorder_insert() */
> +	uint16_t vlan_tci_outer;  /**< Outer VLAN Tag Control Identifier (CPU order) */
> +	struct rte_mbuf *next;    /**< Next segment of scattered packet. */
> +
> +	/* second cache line - fields only used in slow path or on TX */
> +	MARKER cacheline1 __rte_cache_min_aligned;
> +
> +	rss_hash_t hash;      /**< hash information */
> 
>  	union {
> -		uint32_t rss;     /**< RSS hash result if RSS enabled */
> -		struct {
> -			union {
> -				struct {
> -					uint16_t hash;
> -					uint16_t id;
> -				};
> -				uint32_t lo;
> -				/**< Second 4 flexible bytes */
> -			};
> -			uint32_t hi;
> -			/**< First 4 flexible bytes or FD ID, dependent on
> -			     PKT_RX_FDIR_* flag in ol_flags. */
> -		} fdir;           /**< Filter identifier if FDIR enabled */
> -		struct {
> -			uint32_t lo;
> -			uint32_t hi;
> -		} sched;          /**< Hierarchical scheduler */
> -		uint32_t usr;	  /**< User defined tags. See rte_distributor_process() */
> -	} hash;                   /**< hash information */
> +		void *userdata;   /**< Can be used for external metadata */
> +		uint64_t udata64; /**< Allow 8-byte userdata on 32-bit */
> +	};
> +
> +	struct rte_mempool *pool; /**< Pool from which mbuf was allocated. */
> +#else
> +	rss_hash_t hash;      /**< hash information */
> 
>  	uint32_t seqn; /**< Sequence number. See also rte_reorder_insert() */
> 
> @@ -862,7 +887,7 @@ struct rte_mbuf {
> 
>  	struct rte_mempool *pool; /**< Pool from which mbuf was allocated. */
>  	struct rte_mbuf *next;    /**< Next segment of scattered packet. */
> -
> +#endif
>  	/* fields to support TX offloads */
>  	union {
>  		uint64_t tx_offload;       /**< combined for easy fetch */
> --
> 2.8.0.GIT